ifconfig: eliminate trailing whitespace

Eliminate trailing whitespace on inet, inet6, and groups lines. I think the
"list txpower" command will still show some, but I'm not able to test that.

PR: 153731
Reported-by: Nikolay Denev <ndenev@gmail.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D19004